1. Unfold lamb, rinse, and pat dry. Slash 1/2-in.-deep cuts about 1 in. apart all over both sides of lamb. Season all over with 1 1/2 tsp. salt. Put in a baking dish just large enough to hold meat when unfolded.
2. Pour 1/2 cup water and 1/2 cup olive oil into a blender; add onions, garlic, ras el hanout, and remaining 1 1/2 tsp. salt. Pulse to blend into a loose paste and pour over lamb. Cover and refrigerate at least 6 hours or up to 1 day. Let lamb come to room temperature before grilling, about 45 minutes.
3. Prepare a gas grill for indirect heat: Turn all burners to high and close lid. When temperature reaches 325° to 350°, turn off one of the burners, creating an indirect heat area, and adjust burners to maintain temperature. Or preheat oven to 325°.
4. Wipe marinade off lamb and arrange apricot halves in a single layer over meat, leaving about a 1/2-in. border on all sides. Roll up meat jelly-roll style and tie tightly with kitchen twine. Trim off ends of twine. Rub lamb with remaining olive oil. If cooking on grill, oil grill well with oil-soaked paper towels.
5. Grill lamb over indirect heat area or roast in oven, set in a V-rack in a large roasting pan. Cook meat, turning over once halfway through, until an instant-read thermometer registers 140° for medium-rare, 1 1/2 to 2 1/2 hours. Let lamb rest, covered with foil, 15 to 20 minutes. Slice and serve.
Note: Nutritional analysis is per serving.